1. Field of the Invention
The present invention relates to a tire-interior monitoring apparatus, and more particularly to a tire-interior monitoring apparatus for constantly monitoring the tire interior by detecting the physical parameters thereof, such as the air pressure and temperature within the tire of vehicles using pneumatic tires, including automobiles, tire-type streetcars, tire-type construction vehicles, aircraft, and the like.
2. Description of the Related Art
In light of the safe operation of vehicles using pneumatic tires, it is desirable to monitor physical parameters of the tire interior, such as the air pressure and temperature within the tire when the wheels are rotating or stationary. For this purpose, devices for alarming an abnormality of the air pressure of a tire have hitherto been proposed which are adapted to issue an alarm when the air pressure of a tire has declined below a predetermined level (Japanese Patent Application Laid-Open Nos. 51-9859, 50-115980, 52-40383, 62-218208, 62-257039, and 49-128777). The following methods are known among the types of devices for indicating an abnormality of the air pressure of a tire: A resonance method in which a resonance circuit mounted on a tire wheel and including a pressure switch is electromagnetically connected through a coil to a resonance circuit mounted on a chassis to thereby transmit a detected signal, and a method in which a detected signal is transmitted optically. With the apparatus using the resonance method, only small electric power is generated in the resonance circuit mounted on the tire wheel. Therefore, it is difficult to actuate a sensor and active elements mounted on the tire wheel, so that the pressure switch is only turned on and off by a predetermined air pressure of the tire, and it is difficult to constantly monitor the internal air pressure of the tire. In addition, with the optical method, an explosive light-emitting device is made to emit light when the air pressure of the tire has dropped below a predetermined level as to transmit a signal, so as a result, it is difficult to constantly monitor the internal air pressure of the tire.
An apparatus is known for constantly monitoring the air pressure of the tire which employs a mechanically-operated pressure indicator using a Bourdon's tube as well as a bellows which is expanded or contracted with an increase or decrease in the air pressure of the tire (Japanese Patent Application Laid-Open No. 64-32142). However, since the Bourdon's tube and the bellows vibrate as the tire wheel vibrates during its rotation, the detected pressure value varies as a result to the vibrations. Hence, there are drawbacks in that the detection accuracy declines, and that malfunctionings and faults are liable to occur owing to the vibrations.
In addition, another type of signalling apparatus has been proposed in which a power-receiving coil is wound onto an inner periphery of a bead base of a tire rim, and a power-supplying coil is attached to a chassis in face-to-face relation with the power-receiving coil (Japanese National Publication: translated version No. 63-501065 of international application under Patent Cooperation Treaty and U.S. Pat. No. 4,567,459). The arrangement provided is such that an electromotive force is generated in a tire-side section of the apparatus through mutual inductance between the two coils, an electronic circuit on the tire side including active elements is actuated by this electromotive force, and the tire pressure is finally displayed on the instrument panel near the driver seat, or an alarm is issued. With this arrangement, in the case of small tire wheels used in passenger cars or the like, the tire wheels can be handled manually during the mounting and dismounting thereof without causing damage to the power receiving coil wound onto the bead base. However, in the case of large tire wheels used in such as trucks, buses, construction vehicles, streetcars, and aircraft, the tire wheels are large and heavy, so that such tire wheels are in most cases handled by using a special tool during the mounting and dismounting with respect to the axles. Accordingly, it is very difficult to handle such tire wheels without causing damage to the power-receiving coil. In addition, although still another type of monitoring apparatus is known in which electric power is supplied to the tire wheel-side section of the apparatus by using a slip ring (Japanese Patent Application Laid-Open No. 58-12097), since the slip ring becomes worn, it is difficult to supply electric power on a stable basis over extended periods of time. Furthermore, as shown in the publication disclosing the aforementioned abnormality signalling apparatus, it is conceivable to supply electric power to the tire wheel-side sensor by mounting a battery on the tire wheel side, but it is still difficult to monitor the air pressure of the tire on a stable basis over extended periods of time owing to the consumption of the battery.
For this reason, the present inventor has proposed an internal-tire-pressure displaying and/or signalling apparatus capable of constantly monitoring the physical parameters of the tire interior, such as the air pressure, by supplying stable electric power to a detector disposed in the tire. This apparatus has a coupling device for coupling a rotating section to a non-rotating section thereof. The coupling device comprises an inner cylinder and an outer cylinder that are disposed concentrically relative to each other. A group of power supply coils including a power-receiving coil and a power-supplying coil are wound on the inner cylinder and outer cylinder, respectively, so as to supply electric power, and a group of signal transmission coils including a signal-transmitting coil and a signal-receiving coil are wound thereon so as to supply a signal. Since coils are thus wound on the inner and outer cylinders, handling is facilitated as compared with the technique in which the power-receiving coil and the like are wound onto the inner periphery of the aforementioned bead base. In addition, since the group of signal transmission coils and the group of power supply coils are disposed in such a manner as to be spaced apart from each other by a predetermined distance (e.g. 25 mm) in the direction of a rotational axis of the tire wheel, the electromagnetic effect of the group of power supply coils on the group of signal transmission coils, i.e., so-called crosstalk, can be prevented from occurring, thereby transmitting a signal with high accuracy.
However, with the above-described technique proposed by the present inventor, although the above-described advantage can be obtained, the following drawback is encountered: Since the length, in the direction of the rotational axis of the tire wheel, of the coupling device for coupling a rotating section and a non-rotating section of the apparatus is made long to prevent crosstalk, it is difficult to mount the apparatus on a vehicle limited in the vehicle width or a vehicle not provided with a sufficient clearance between a mounting surface on the rotating side and a mounting surface on the non-rotating side, such as special tire-type electric vehicles and aircraft.